Flat-panel display devices are widely used in conjunction with computing devices, in portable devices, and for entertainment devices such as televisions. Such displays typically employ a plurality of pixels distributed over a substrate to display images. Each pixel incorporates several, differently colored, light-emitting elements commonly referred to as sub-pixels, typically emitting red, green, and blue light, to represent each image element. A variety of flat-panel display technologies are known, for example plasma displays, liquid crystal displays, and light-emitting diode displays. In recent years, there has been an increased interest in flexible flat-panel display devices.
Light emitting diodes (LEDs) incorporating thin films of light-emitting materials forming light-emitting elements (pixels) have many advantages in a flat-panel display device and are useful in optical systems. U.S. Pat. No. 6,384,529 to Tang et al. shows an organic LED (OLED) color display that includes an array of organic LED light-emitting elements. Alternatively, inorganic materials can be employed and can include phosphorescent crystals or quantum dots in a polycrystalline semiconductor matrix. Other thin films of organic or inorganic materials can also be employed to control charge injection, charge transport, or charge blocking to the light-emitting-thin-film materials, and are known in the art. The materials are placed upon a substrate between electrodes, with an encapsulating cover layer or plate. Light is emitted from a sub-pixel when current passes through the light-emitting material. The frequency of the emitted light is dependent on the nature of the material used. In such a display, light can be emitted through the substrate (a bottom emitter) or through the encapsulating cover (a top emitter), or both.
Two different methods for controlling the pixels in a flat-panel display device are generally known: active-matrix control and passive-matrix control. In an active-matrix device, control elements are distributed over the flat-panel substrate. Typically, each sub-pixel is controlled by one control element and each control element includes at least one transistor. For example, in a simple, prior-art active-matrix organic light-emitting (OLED) display, each control element includes two transistors (a select transistor and a power transistor) and one capacitor for storing a charge specifying the luminance of the sub-pixel. Each light-emitting element typically employs an independent control electrode and a common electrode.
One common, prior-art method of forming active-matrix control elements typically deposits thin films of semiconductor materials, such as silicon, onto a glass substrate and then forms the semiconductor materials into transistors and capacitors through photolithographic processes. The thin-film silicon can be either amorphous or polycrystalline. Thin-film transistors (TFTs) made from amorphous or polycrystalline silicon are relatively large and have lower performance compared to conventional transistors made in crystalline silicon wafers. Moreover, such thin-film devices are formed at high temperatures (e.g. >300° C. for amorphous silicon and >500° C. for large-grain polysilicon) and typically exhibit local or large-area non-uniformity across the glass substrate that results in non-uniformity in the electrical performance and visual appearance of displays employing such materials. Thin-film transistors made of organic materials (OTFTs) have lower performance than silicon TFTs and are more susceptible to moisture but are processed at lower temperatures (e.g. <200° C.) (e.g. U.S. Patent Application Publication No. 2006/0163559). All TFTs, however, require a very smooth substrate to avoid forming inoperable or dysfunctional elements.
Flexible substrates made of plastic are relatively transparent but have many limitations. Such substrates typically physically degrade at temperatures greater than 300° C. or even at 250° C. or 200° C. Hence processing the substrate, or materials deposited on the substrate, is very difficult. For example, conventional PEN has a process temperature of approximately 150° C. and PET has a process temperature of approximately 120° C. Furthermore, plastics can have limited resistance to chemicals and processes used in conventional photolithography, thereby limiting the kinds of materials and processing employed with the substrates. Such substrates are also know to have poor dimensional stability with varying process temperatures, stress, and relative humidity, thereby limiting the resolution of structures formed on the substrate. Plastic substrates are also subject to permeation of oxygen and water vapor that can degrade organic materials such as OLED materials. Moreover, as noted in U.S. Pat. No. 7,466,390, it is difficult to form plastics that are smooth and clean enough to serve as a substrate for the formation of thin-film electronic elements, such as those commonly used for flat-panel displays, e.g. thin-film silicon transistors. Catalytic byproducts and inorganic particulates in the polymers and inadequate process conditions all affect surface roughness. For soft materials such as polymers, artifacts such as scratches or foreign contaminants can be problematic.
Commercial flexible substrate and cover products incorporating plastics with multi-layer barriers are now available that have a greatly reduced susceptibility to moisture permeation, for example from 3M. Such multi-layers typically employ alternating organic and inorganic layers. Plastics can also be heat stabilized to improve their dimensional stability and temperature range (see, e.g. U.S. Pat. No. 7,449,135). Such products, however, are still relatively rough, face many of the difficulties listed above, and are inadequate for thin-film display devices using thin-film transistors without additional stabilization and surface treatments.
An alternative approach to providing flexible substrates is to use metal foils. Metal foils have the advantages of moisture impermeability, a lower coefficient of thermal expansion, and are relatively inexpensive as well as compatible with high process temperatures of, for example 900° C. Metal foils, for example steel or aluminum foil, however, are opaque and thus cannot serve as both a substrate and cover in a flat-panel display device (since light must escape from a flat-panel display). Metal foils are also very rough and require a smoothing process or the use of a planarization layer or annealing processes to provide a surface with adequate smoothness to support the formation of thin-film electronic components (e.g. as described in U.S. Pat. No. 7,037,352). For example, typical steel foil has an rms (root mean square) roughness of >600 Angstroms. By forming an inorganic layer of silicon dioxide by PECVD on the steel foil surface, the rms roughness can be reduced in half, for example to >300 Angstroms. Other techniques, for example chemical or mechanical polishing, can reduce the rms roughness by more than an order of magnitude, for example >20 Angstroms. Further treatments can reduce the rms roughness to >10 Angstroms and can be as smooth as the commercially available substrate glass, e.g. Corning Eagle 2000. An rms surface roughness of less than 10 Angstroms is adequate for thin-film transistor formation. Steel foils can also be planarized by using spin-on-glass techniques, possibly employing several layers and materials such as polyimide. Hence, metal foil substrates can be made smooth enough to support conventional thin-film transistor processing and performance.
In general, for both plastic and metal substrates, the thickness of the planarizing coatings is less than one micron. However, such smoothing processes are expensive and time consuming. Moreover, the costs of the substrates increase if additional treatments to stabilize the substrate materials are necessary.
Matsumura et al describe crystalline silicon substrates used for driving LCD displays in US Patent Application Publication No. 2006/0055864. The application describes a method for selectively transferring and affixing pixel-control devices made from first semiconductor substrates onto a second planar display substrate. Wiring interconnections within the pixel-control device and connections from busses and control electrodes to the pixel-control device are shown. However, these substrates are relatively thick and not well adapted to flexible devices.
There is a need, therefore, for an emissive display device having improved performance and flexibility and reduced manufacturing process and material requirements.